There is an increasing need for mobile high-speed communication systems to provide a variety of services such as downloading music files, TV, Internet, and photo sharing. A mobile high-speed communication system must overcome many difficult operating conditions. Among the many conditions the system must contend with are interference, multipath signals, changing obstructions to the signal line-of-site, Doppler shift, inter-symbol interference (ISI), and changing distances between transmitter and receiver. Orthogonal Frequency Division Multiplexing (OFDM) is one technique developed for high-speed communications that can mitigate many of these difficult conditions.
OFDM divides an allocated communication channel into a number of orthogonal subchannels of equal bandwidth. Each subchannel is modulated by a unique group of subcarrier signals, whose frequencies are equally and minimally spaced for optimal bandwidth efficiency. The group of subcarrier signals are chosen to be orthogonal, meaning the inner product of any two of the subcarriers equals zero. An inverse fast Fourier transform (IFFT) is often used to form the subcarriers. The number of orthogonal subcarriers determines the fast Fourier transform (FFT) size (N) to be used.
Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user version of OFDM. For a communication device such as a base station (BS), multiple access is accomplished by assigning subsets of the orthogonal sub-carriers to individual subscriber devices, such as mobile stations (MS), with which the base station is communicating. OFDMA may be considered to be a combination of frequency and time domain multiple access, where a time-frequency space is partitioned and the mobile station data is assigned along the OFDM symbols and subcarriers.
In telecommunications, a frame is a fixed or variable length packet of data, encoded in accordance with a communication protocol for digital transmission. A frame structure defines the way a multiplexer divides a communication channel into frames for transmission. The frame structure of an OFDM or OFDMA system has a major impact on the performance of the system. Currently, there is limited choice for high performance OFDM and OFDMA frame structures. Therefore, there is a need for systems and methods that provide a flexible frame structure for high performance OFDM and OFDMA systems.
In particular, the 802.16e amendment to the IEEE 802.16 standard, which is referred to as “802.16e” or simply as “16e” herein, has defined a relatively rigid frame structure in accordance with the WirelessMAN-OFDMA Reference System. A new amendment to the IEEE 802.16 standard, the 802.16m amendment, which is referred to as “802.16m” or simply as “16m” herein, has been proposed. Requirements for the development of the 802.16m as specified by IEEE 802.16m System Requirements Document (IEEE 802.16m SRD), IEEE 802.16m-07/002r4, Oct. 19, 2007, which is incorporated by reference herein in its entirety, stipulate many improvements in performance over the 802.16e WirelessMAN-OFDMA Reference System and operation in many different deployment environments. Improvements in performance include reductions in latency across the air interface, increases in user and sector throughput, and reductions in system overhead. Operation is also required in the presence of varying levels of mobility, from stationary up to 350 km/h and beyond, and in sectors and cells with drastically different coverage ranges, from micro-cells and even femto-cells with coverage ranges in the 10's to 100's meters to large rural macro-cells with coverage ranges greater than 5 kilometers.
The relatively rigid frame structure that currently exists in IEEE 802.16e operating with the OFDMA physical layer is unlikely to maximize the achievable performance under such diverse deployments and operational conditions. Therefore, there is a need for a more flexible frame structure that allows maximal performance to be more readily achieved under the given deployment and operational conditions.
An added constraint on the system design of IEEE 802.16m is the requirement to support legacy Mobile Stations (MS) that conform to IEEE 802.16e WirelessMAN-OFDMA Reference System on the same radio frequency carrier simultaneously with IEEE 802.16m MSs. In this mixed mode of operation, the legacy MSs must be able to operate as if they were being served by a Base Station (BS) that conforms only to the WirelessMAN-OFDMA Reference System. Therefore, there is also a need for an IEEE 802.16m frame structure that provides support for legacy MSs under IEEE 802.16e.